Coma Cluster Treasury Survey

Project Overview - Objective 3

The Global Structure and Scaling Laws of Dwarf Galaxies

Despite being the most numerous type of galaxy in nearby clusters, dwarf galaxies are among the most poorly studied due to their faint, low surface brightness levels (22 < μe < 26 B mag arcsec-2). Like their more massive counterparts, dwarf galaxies exhibit well-defined correlations between their various physical (global) parameters, such as luminosity, size, surface brightness, colour, internal dynamics, and chemical abundances. These so-called "scaling laws" reveal a great deal about the physical processes which operated during the early stages of galaxy formation, of which major contributors are feedback from star-formation, gravitational (tidal) interactions with other galaxies, and interactions with the hot intergalactic gas which encompasses clusters.

The study of these "scaling laws" in various environments (e.g., a densely populated cluster versus the isolation of field galaxies) provides the key observational reference needed to test specific predictions of current theoretical models of dwarf galaxy formation and evolution. For instance, the "galaxy harassment model" predicts that both dwarf elliptical (dE) and dwarf lenticular (dS0) galaxies in the highest-density cluster regions should have steeper light profiles, a higher fraction will be nucleated, the fraction of any remaining disk structure should be lower, and they should have higher metallicities than those located in lower-density environments. On the other hand, "hierarchical models" predict old ages and low metallicities for dE/dS0 galaxies in high-density environments. Alternatively, younger ages and higher metallicities in lower-density cluster environments would support their evolution from accreted field galaxies.

Our team is acquiring chemical abundance measurements, known as line strength indices, and internal dynamical measurements, known as velocity dispersions, for hundreds of dE/dS0 galaxies in the core and infall region of the Coma cluster. Together with measurements of the galaxy sizes and brightnesses from the HST images, we can perform a comprehensive study of the "scaling laws" for dE/dS0s galaxies. Comparison between the slope and scatter of these scaling laws for the Coma core, Coma infall region, lower density clusters, and field samples will enable one to test for any environmental effects on the structural and stellar population properties of dwarf galaxies.

Ultra Compact Dwarf galaxies

We will also search for a possible population of Ultra Compact Dwarf (UCD) galaxies; a relatively new addition to the family of small stellar systems. To date they have been detected in the less dense Fornax and Virgo galaxy clusters. They range in brightness from MV = -13.5 mag down to the magnitudes of the brightest globular clusters. Their origin, and indeed their status as a separate class of object, remains unknown. A study of the abundance, structure and color of UCDs in a richer environment should help to constrain the viability of potential formation models.